The water-steam circuit of a power plant is normally cleaned by chemical means before being taken into service. Either acids or EDTA (ethylenediaminetetraacetic acid) are used for this purpose. These chemicals must be removed from the system again upon completion. At least four times the system volume (up to 1000 m3) is necessary for this in a typical acid cleaning operation. This gives rise to two problems:
1. The total volume must be treated as wastewater or disposed of. In light of increasingly stringent statutory requirements this leads to high costs.
2. The post-flushing process is susceptible in terms of execution and drainage if not carried out correctly. The risk of renewed corrosion is great.
For this reason the wetting agent step and the pickling step are usually combined during the acid cleaning operation, whereby the pickling solution is not drained off and subsequently is replaced wet on wet by deionized water and thereafter passivated. Water consumption is high, however.
EP 0 273 182 discloses a method for cleaning a vessel, wherein iron oxide present in the vessel is dissolved by means of a complex-forming acid, and the iron oxide is removed as a dissolved iron complex by emptying of the vessel. In order to prevent air ingress during emptying, the vessel is evacuated by means of an inert gas, in particular by means of nitrogen or water vapor, which is injected under pressure.